Preheat your oven to 375°F (190°C). Slice the tops off the peppers and remove the seeds and membranes, creating a hollow cavity in each.
In a mixing bowl, combine the cooked grains, diced tomatoes, chopped vegetables, chopped herbs, and minced garlic. Drizzle with olive oil and season with salt and pepper, then stir until well mixed.
Stuff each pepper generously with the veggie and grain mixture, pressing down slightly to pack the filling in.
Place the stuffed peppers upright in a baking dish. Sprinkle the tops with nutritional yeast or vegan cheese for a cheesy crust.
Bake uncovered in the preheated oven for about 30 minutes, until the peppers are tender and the topping is golden and bubbly.
Remove from the oven and let cool slightly. Serve hot, garnished with extra herbs if desired, for a colorful and hearty vegan main dish.